Central anatomical correlates of aging and age-related hearing loss (presbycusis) will be studied in two mouse models: the C57BL/6J, which demonstrates progressive sensorineural hearing loss (SNHL) with onset during young adulthood to middle age; and the CBA/J, which maintains good auditory sensitivity until late in life. Several age groups will be studied, ranging from young (2-months old) to very old (over 30-months). Two groups of CBA mice with induced chronic SNHL (similar to that occurring in C57 mice) will also be employed. Quantitative morphometric evaluations will be made in subdivisions of the cochlear nucleus (CN) and inferior colliculus (IC) using Nissl-stained, fiber-stained, and Golgi- impregnated tissue. A variety of cellular features will be assessed including cell number (neurons and glia), neuronal soma size and shape, dendritic dimensions and orientation, presence of spines, signs of degeneration, etc. Data will be statistically analyzed as a function of age, brainstem subnucleus, and when appropriate, cell type or location with regard to tonotopic organization. Peripheral input to the brainstem will be evaluated using quantitative counts of spiral ganglion cells. The final product will be a comprehensive, detailed anatomical description of two major auditory brainstem regions in two mouse models of presbycusis. In addition, by comparisons among data from the various subject groups the contributions of aging per se, the occurrence and chronicity of SNHL, and genotype will be elucidated.